Wireless electrical energy transmission from a Power Transmitting Unit (PTU) to a Power Receiving Unit (PRU) can be achieved without man-made conductors. This mode of power transmission may be useful in a number of circumstances, for example, when the interconnection wires are inconvenient, hazardous, or impossible. Cross connection occurs when the radio-frequency (RF) control channels such as Bluetooth Low Energy (BTLE) forms a connection between wrong PTU-PRU pairs. Currently, there exists a mechanism for detection of PRU impedance change by triggering a 105 ms discovery window for an initial advertisement form the PRU. Another mechanism uses BTLE received signal strength indication (RSSI) filtering to try and correlate the advertisement transmitter with proximity to a charging surface.
Detection of a PRU during the short beacon by methods of impedance detection is quite difficult or impossible in some cases and remains an unresolved issue. Some solutions remove the requirement that a PTU has to detect a PRU using short beacons. Other solutions require and test that PTUs can detect a minimum reflected impedance change from an open pad, which are not clear regarding how to create the reflected impedance. There are also proposals for requiring and testing that PTU has the capability to detect a special PRU reflected impedance. The PTU periodically (e.g., every ¼ second) transmit a short (e.g., 10 msec long) beacon to detect a presence of a PRU, and once the presence of the PRU is reliably detected, the PTU can further energize its coil and transmit a stronger and longer charging pulse (e.g., a long beacon). The PTU saves a substantial amount of energy by not having to transmit long beacons very often.